Communications system using hierarchical queue structure for email message delivery and related methods

ABSTRACT

A communications system may include at least one destination server for hosting email message boxes, and communications devices for generating email messages each associated with a respective message box. A delivery server may have a plurality of queues and a controller. The controller may store the email messages in a first queue, and attempt to send the stored email messages to the destination server at a first sending rate. The controller may also move email messages stored in the first queue to a second queue based upon a delivery failure. Email messages may be sent from the second queue at a second sending rate less than the first sending rate. The controller may also move email messages from the second queue to the first queue having a common characteristic with a successfully delivered email message.

FIELD OF THE INVENTION

The present invention relates to the field of communications systems,and, more particularly, to electronic mail (email) systems and relatedmethods.

BACKGROUND OF THE INVENTION

In an email system, email messages are typically generated by users oncommunications devices such as personal computers (PCs), personal dataassistants (PDAs), etc. Once generated, these messages are forwarded toan email delivery server which hosts the users' email accounts. Forexample, in a corporate environment, a user's communications device maybe connected to an email delivery server via a local area network (LAN),while home users may connect to a delivery server of an Internet serviceprovider (ISP) via a dial-up or broadband communications link.

The email delivery server temporarily stores the email messages in aqueue for processing and sending to the destination message box (orboxes) associated with the email message. The destination message box isdesignated by the recipient address the user includes with the email.Presuming that a recipient's message box is not hosted on the emaildelivery server, the email messages are forwarded via a wide areanetwork (WAN) (e.g., the Internet) to a destination server at which therecipient message box is hosted. The email address associated with themessage also designates the appropriate destination server. Once thedestination server receives an email message, it stores the emailmessage in the intended recipient's message box.

Sometimes problems arise during this email delivery process. Perhaps themost frequent of these problems is that a recipient's message box isfull, in which case the destination server is unable to store new emailmessages in the message box. In such case, the destination server willtypically generate a fatal error message, which is returned to the emaildelivery server to indicate that the message has not been delivered.Another potential problem is that the destination server is down orotherwise unavailable. In this case, an intermediate server or routingdevice will generate a temporary failure message, which is also returnedto the email delivery server.

The email delivery server sends email messages stored in its queue at apredetermined sending rate (every few minutes, etc.). When a largenumber of mail delivery failures occur, the queue begins to fill up asusers continue to generate more and more messages and the undeliveredmessages remain in the queue. Not only does this consume large amountsof storage space, but it also consumes significant processing resourcesas the email delivery server will repeatedly try to send the messagesuntil it no longer receives a failure message.

One prior art approach to alleviating this problem is to implement asecondary queue at the email delivery server, which is sometimesreferred to as a spillover or fallback queue. When a delivery failure isreceived for a particular email message stored in the delivery server'sprimary queue, the email message is then moved to the secondary queue.The email delivery server may then attempt to send messages stored inthe secondary queue at a lower sending rate than the primary queue. Thishelps conserve system resources in that the email delivery server is notspending as much time trying to re-send the failed messages, and it doesnot have to access the primary queue as often. Two examples of emailserver systems which use secondary queues are the Sendmail High VolumeMail Solution (HVMS), and Cisco System's E-mail Manager.

Despite the advantages of such systems, additional flexibility formanaging primary and secondary queues of email delivery servers may bedesirable in certain applications.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a communications system which providesenhanced email queue management features and related methods.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a communications system which mayinclude at least one destination server for hosting a plurality ofelectronic mail (email) message boxes, and a plurality of communicationsdevices for generating email messages each associated with a respectivemessage box. The system may further include a delivery server includinga plurality of queues and a controller.

More particularly, the controller may be for storing the email messagesgenerated by the communications devices in a first queue, and attemptingto send the stored email messages to the at least one destination serverat a first sending rate. The controller may also move email messagesstored in the first queue to a second queue based upon a deliveryfailure. The controller may then attempt to send email messages storedin the second queue to the at least one destination server at a secondsending rate less than the first sending rate. The controller may alsoadvantageously move email messages from the second queue to the firstqueue having a common characteristic with a successfully delivered emailmessage.

By way of example, the delivery failures may be based upon a failure todeliver email messages to respective message boxes, and the commoncharacteristic may thus be a common message box. Moreover, the at leastone destination server may be a plurality of destination servers.Accordingly, the delivery failures may be based upon a failure todeliver email messages to respective destination servers, and the commoncharacteristic may be that the email messages have respective messageboxes hosted by a common destination server. Thus, the controlleradvantageously moves email messages to the first queue for quickerdelivery to the appropriate destination server once it has beendetermined that the destination server or message box is able to receiveemail messages.

Furthermore, the controller may store directly in the second queue emailmessages generated by the communications devices which share the commoncharacteristic with an email message already stored in the second queue.Thus, for example, if a new message is received from a communicationsdevice that has the same destination server and/or message boxassociated therewith as a message already moved to the second queue,then the message may be stored directly in the second queue. That is,since it is already known that attempts to send email messages to thedestination server and/or message box in question have previouslyfailed, there is no need to fill the first queue with a new messagewhose delivery will most likely fail for the same reason.

The second queue may also be a plurality thereof arranged in a hierarchyeach having a respective storage interval associated therewith. Further,the storage intervals may successively increase from a highest queue inthe hierarchy to a lowest queue. The controller may thus move emailmessages to one of the queues in the hierarchy (e.g., the highest queue)based upon a delivery failure. Also, the controller may move emailmessages stored in a higher queue in the hierarchy to a next lower queuein the hierarchy after being stored in the higher queue for therespective storage interval thereof. Moreover, the controller mayattempt to send messages from each of the queues in the hierarchy atsuccessively decreasing sending rates from the highest queue to thelowest queue. Further, the controller may discard messages from thelowest queue in the hierarchy after being stored therein for the storageinterval thereof.

By way of example, one or more of the plurality of communicationsdevices may be a wireless communications device. Also, thecommunications system may further include a wide area network (WAN),such as the Internet, connecting the at least one destination server andthe delivery server.

An email communications method aspect of the invention may includehosting a plurality of email message boxes on at least one destinationserver, and generating email messages each associated with a respectivemessage box. The method may further include storing the email messagesin a first queue, and attempting to send the stored email messages tothe at least one destination server at a first sending rate.Additionally, email messages stored in the first queue may be moved to asecond queue based upon a delivery failure thereof. The method may alsoinclude attempting to send email messages stored in the second queue tothe at least one destination server at a second sending rate less thanthe first sending rate, and moving email messages from the second queueto the first queue having a common characteristic with a successfullydelivered email message.

Other advantageous aspects of the invention relate to a delivery server,such as the one described briefly above, and a related computer-readablemedium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a communications system inaccordance with the present invention.

FIG. 2 is a schematic block diagram of an alternate embodiment of thecommunications system of FIG. 1.

FIG. 3 is flow diagram illustrating an email communications method inaccordance with the present invention.

FIG. 4 is flow diagram illustrating an alternate embodiment of the emailcommunications method of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime notation is used toindicate similar elements in alternate embodiments.

Referring initially to FIG. 1, a communications system 20 in accordancewith the present invention illustratively includes a destination server21 for hosting a plurality of email message boxes (MB) 22 a-22 n. Whileonly a single destination server 21 is shown in the illustrated examplefor clarity of illustration, it will be appreciated by those skilled inthe art that numerous destination servers may be used, as will bediscussed further below. Further, the system 20 also illustrativelyincludes a plurality of communications devices 23-25 for generatingemail messages each associated with a respective one of the messageboxes 22 a-22 n. By way of example, the communications devices 23-25 maybe laptop computers, personal data assistants (PDAs), and desktoppersonal computers (PC), as illustratively shown. Other suitable devicesfor generating emails may also be used, as will be appreciated by thoseskilled in the art.

A delivery server 26 receives the emails from the communications devices23-25 and sends them to the destination server 21. In the illustratedexample, the delivery server 26 and destination server 21 communicatevia a wide area network (WAN) 27, such as the Internet, for example. Thecommunications devices 23-25 may be connected with the delivery server26 in a local area network (LAN), for example, including a wireless LAN(WLAN) in some embodiments, as will be appreciated by those skilled inthe art.

In particular, the delivery server 26 illustratively includes acontroller 30, a first (or primary) queue 31, and a second queue 32. Thecontroller 30 stores the email messages generated by the communicationsdevices 23-25 in the first queue 31. The controller 30 then attempts tosend the email messages stored in the first queue 31 to the destinationserver 21 at a first sending rate. Typically, messages will be sent fromthe first queue 31 at a relatively fast rate, such as every few minutesor less. The controller 30 may be implemented as a software module whichruns on the delivery server 26, and the queues 31, 32 may be implementedas database modules, for example.

Once received by the destination server 21, the destination serverdelivers the email messages to their respective message boxes 22 a-22 n.As will be appreciated by those skilled in the art, an address isassociated with an email message when it is generated. The addressidentifies the message box 22 for which the message is intended, as wellas the destination server 21 hosting the message box. Depending upon agiven implementation, the destination server 21 may return aconfirmation indication to the delivery server 26 to indicate that anemail message has been successfully delivered to the appropriate mailbox22.

Yet, as discussed above, it is not uncommon for email message deliveryto fail for a variety of different reasons. By way of example, messageboxes 22 a-22 n hosted on a corporate or Internet service provider's(ISP's) server will typically be allotted a limited amount of storagespace. If a user does not empty his respective message box 22, he willeventually exceed his allotted storage space, and the delivery server 21will then stop storing messages for that user until space is once againmade available. If a message box 22 is full and a new message arrives atthe destination server 21 for this message box, the destination serverwill return a failure message to the delivery server 26. By way ofexample, a failure message resulting from a full mailbox may take theform of a fatal error data source name (DSN) code, as will beappreciated by those skilled in the art.

Another reason for a delivery failure may be that the destination server21 is down or otherwise unavailable. In this case, a server or otherrouting device within the WAN 27 may generate a failure message for thedelivery server 26 letting it know that the destination server 21 isunavailable. For example, the delivery server 26 may receive a temporaryfailure DSN code in such an event.

When the controller 30 receives one of the above types of deliveryfailure messages (or others) for a given email message, the controllerthen advantageously moves the message to the second queue 32. Thispromotes system resource savings in several ways. First, the controllerattempts to send email messages stored in the second queue 32 to thedestination server 21 at a second sending rate less than the firstsending rate. Thus, the controller 30 does not have to devote as muchtime to re-sending these messages as it otherwise would if they werestill in the first queue 31. Moreover, the second queue may be storedwithin a different directory or location of the delivery server 26.Thus, the number of operations that require access to the first queue 31at any given time is reduced, which helps reduce bottlenecks, as will beappreciated by those skilled in the art.

In accordance with the invention, the controller 30 may alsoadvantageously move email messages from the second queue 32 to the firstqueue 31 having a common characteristic with a successfully deliveredemail message. More particularly, if an email message had been moved tothe second queue 32 because its respective message box 22 was full, thesuccessful delivery of another email message to that same message boxwould indicate to the controller 30 that the message box was again ableto receive messages. Accordingly, the controller 30 would move the emailmessage from the second queue 32 to the first queue 31 so that it wouldbe delivered more quickly to the given message box 22.

Similarly, if the delivery failure is a result of a destination server21 failure, then the controller 30 may then move email messages whichwere stored in the second queue 32 because of the destination server'sunavailability to the first queue 31 once it is determined that thedestination server is once again available. This embodiment isparticularly advantageous where there are numerous destination servers41′, 51′ for which delivery failure messages may be received, as shownin FIG. 2. In this exemplary embodiment, the destination servers 41′,51′ and their respective messages boxes 42 a′-42 n′, 52 a′-52 n′ aresimilar to the destination server 21 and messages boxes 22 a-22 n notedabove.

Moreover, in the present embodiment the delivery server 26′ communicateswith wireless communications devices 35′, 36′ via a wirelesscommunications network 37′. By way of example, the wirelesscommunications network 37′ may be a cellular network, and the wirelesscommunications devices 35′, 36′ may be cellular-based email devices suchas Blackberry devices or email ready cellular telephones, as shown.

To provide still further efficiencies, the single second queue 32described above has been replaced in the present embodiment with aplurality of queues 32 a′-32 c′ arranged in a hierarchy (HQs), as shown.Here, the queue 32 a′ is the highest queue in the hierarchy, the queue32 b′ is the intermediate queue, and the queue 32 c′ is the lowestqueue. Each queue 32 a′-32 c′ has a respective storage intervalassociated therewith, and these intervals successively increase from thehighest queue 32 a′ to the lowest queue 32 c′.

Thus, when an email message has been stored in a given one of the queues32 a′-32 c′ for the respective storage interval thereof, the controller30′ moves the email message to the next lower queue in the hierarchy.This helps further relieve congestion over the use of a single secondqueue, as email messages are then distributed over multiple queuesinstead of all collecting in one. Of course, the number of queues usedin a given hierarchical implementation will vary depending upon theemail message volume being handled and other considerations, as will beappreciated by those skilled in the art.

One exemplary embodiment would be to include seven second queues whichrespectfully store email messages up to intervals of one hour, twohours, four hours, eight hours, twelve hours, twenty-four hours, andforty-eight hours after they are received by the controller 30′. Once anemail message has remained in the lowest queue in the hierarchy for thestorage interval thereof, the controller 30′ may discard the message,for example. Of course, other actions could be taken, such as notifyingthe user who generated the email message of its failure to be delivered,etc., as will be appreciated by those skilled in the art.

The controller 30′ may also attempt to send messages from each of thequeues 32 a′-32 c′ at successively decreasing sending rates from thehighest queue 32 a′ to the lowest queue 32 c′. Accordingly, the emailmessages do not have to be processed as often once they are moved to anext lower queue in the hierarch, as discussed above. Various sendingrates may be used based upon the given implementation.

In accordance with another particularly advantageous feature of theinvention, the controller 30′ may store directly in any one of thequeues 32 a′-32 c′ email messages generated by the communicationsdevices 35′, 36′. This would be done when such messages share a commondestination server 41′, 51′ and/or message box 42′, 52′ with an emailmessage already stored in one of the queues 32 a′-32 c′, where suchdestination server or message box was the reason for the unsuccessfuldelivery of the stored message.

Thus, for example, if a new message is received from the communicationsdevice 35′ that has the same destination server and/or message boxassociated therewith as a message already in one of the queues 32 a-32b′, then the message is stored directly in one of these queues. Thiscould be the highest queue 32 a′, or the same queue in which the emailmessage with the common characteristic is already stored, for example.

It should be noted that rather than restoring a message to the firstqueue 31′ after a successful delivery of another email message having acommon characteristic therewith, as described above, the message couldinstead be moved to another in the hierarchy (or other queue), ifdesired. Moreover, the controller 30′ could check multiple commoncharacteristics (e.g., server failure, message box failure, etc.) indetermining whether to move email messages to the first queue 31′. Thus,for example, if it was determined that a given delivery server was onceagain available, the controller 30′ may move all email messages storedin the queues 32 a′-32 c′ to the first queue 31′, except those for whichdelivery failed because of a message box delivery failure. Also, thesuccessful delivery of a message need not be from the first queue 31′,but it could be from any of the queues 32 a′-32 c′ as well.

An email communications method aspect of the invention will now bedescribed with reference to FIG. 3. Beginning at Block 60, emailmessages are generated by the communications devices 23-25 (Block 61),each of which is associated with a respective message box 22 a-22 n. Theemail messages are stored in the first queue 31 (FQ) and an attempt ismade to the destination server 21 at a first sending rate, at Block 62.Additionally, email messages stored in the first queue 31 are moved tothe second queue 32 (SQ) based upon a delivery failure, at Blocks 63-64.

The method further illustratively includes attempting to send emailmessages stored in the second queue 32 to the destination server 21 at asecond sending rate less than the first sending rate, at Block 65.Further, email messages are moved from the second queue 32 to the firstqueue 31 having a common characteristic with a successfully delivered(Block 66) email message, at Block 67, as discussed above, thusconcluding the illustrated method (Block 68).

Turning additionally to FIG. 4, further method aspects of the inventionwhich may be performed using the hierarchical second queue structureillustrated in FIG. 2 are now described. As noted above, the controller30′ may determine that a newly received email message has a commoncharacteristic(s) with another message already stored in one of thehierarchical queues 32 a′-32 c′, at Block 70′. If this is the case, thecontroller 30′ stores the email message in the highest queue 32 a′ inthe hierarchy, for example, at Block 71′.

If no successful delivery of another email message sharing a commoncharacteristic occurs (Block 66′), the controller 30′ attempts to sendthe messages from the queue 32 a′ during the storage interval thereof,at Blocks 72′-73′. The email messages are successively moved down thehierarchy of queues 32 a′-32 c′ and the foregoing steps are repeated,until the lowest queue 32 c′ is reached and the storage interval thereofexpires, at Blocks 74′-75′, at which point the email messages arediscarded (Block 76′). Of course, if at any point before discarding asuccessful delivery occurs, the stored message(s) sharing the commoncharacteristic is moved to the first queue 31′ and sent (Block 77′) asdescribed above.

Other advantageous aspects of the invention relate to a delivery server26, such as the one described briefly above, and a relatedcomputer-readable medium for performing the steps described above withreference to FIGS. 3 and 4.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A communications system comprising: at least one destination serverfor hosting a plurality of electronic mail (email) message boxes; aplurality of communications devices for generating email messages eachassociated with a respective message box; and a delivery servercomprising a plurality of queues and a controller and a memorycooperating therewith for storing the email messages generated by thecommunications devices in a first queue, and attempting to send thestored email messages to the at least one destination server at a firstsending attempt rate, moving email messages stored in the first queue toa second queue based upon receipt of a delivery failure message,attempting to send email messages stored in the second queue to the atleast one destination server at a second sending attempt rate that isless than the first sending attempt rate, the second queue being one ofa plurality of queues arranged in a hierarchy, each queue in theplurality of queues having a storage interval that successivelyincreases from a highest queue to a lowest queue, the storage intervalbeing independent from the first and second sending attempt rates,moving email messages from a higher queue to a next lower queue afterbeing stored in the higher queue for a duration of its storage interval,and moving email messages having a common characteristic with asuccessfully delivered email message to the first queue.
 2. Thecommunications system of claim 1 wherein the delivery failure message isbased upon a failure to deliver an email message to a respective messagebox; and wherein the common characteristic comprises a common messagebox.
 3. The communications system of claim 1 wherein the at least onedestination server comprises a plurality of destination servers; whereinthe delivery failure message is based upon a failure to deliver emailmessages to the destination servers; and wherein the commoncharacteristic comprises having respective message boxes hosted by acommon destination server.
 4. The communications system of claim 1wherein the controller stores directly in the second queue emailmessages generated by the communications devices sharing the commoncharacteristic with an email message already stored in the second queue.5. The communications system of claim 1 wherein the controller attemptsto send messages from each of the queues in the hierarchy atsuccessively decreasing sending rates from the highest queue to thelowest queue.
 6. The communications system of claim 1 wherein thecontroller discards messages from the lowest queue in the hierarchyafter being stored therein for the storage interval thereof.
 7. Thecommunications system of claim 1 wherein at least one of the pluralityof communications devices comprises a wireless communications device. 8.The communications system of claim 1 further comprising a wide areanetwork (WAN) connecting the at least one destination server and thedelivery server.
 9. A delivery server for delivering electronic mail(email) messages from a plurality of communications devices to at leastone destination server hosting a plurality of email message boxes, eachemail message being associated with a respective message box, thedelivery server comprising: a plurality of queues arranged in ahierarchy, each queue in the plurality of queues having a storageinterval associated that successively increases from a highest queue toa lowest queue; and a controller and a memory cooperating therewith forstoring the email messages generated by the communications devices in afirst queue, and attempting to send the stored email messages to the atleast one destination server at a first sending attempt rate, the firstsending attempt rate being independent from the respective storageinterval, moving email messages stored in the first queue to a secondqueue based upon receipt of a delivery failure message, attempting tosend email messages stored in the second queue to the at least onedestination server at a second sending attempt rate that is less thanthe first sending attempt rate, the second attempt rate beingindependent from the respective storage interval, the second queue beingone of the plurality of queues, moving email messages from a higherqueue to a next lower queue after being stored in the higher queue for aduration of its storage interval, and moving email messages having acommon characteristic with a successfully delivered email message to thefirst queue.
 10. The delivery server of claim 9 wherein the deliveryfailure message is based upon a failure to deliver an email message to arespective message box; and wherein the common characteristic comprisesa common message box.
 11. The delivery server of claim 9 wherein the atleast one destination server comprises a plurality of destinationservers; wherein the delivery failure message is based upon a failure todeliver email messages to the destination servers; and wherein thecommon characteristic comprises having respective message boxes hostedby a common destination server.
 12. The delivery server of claim 9wherein the controller stores directly in the second queue emailmessages generated by the communications devices sharing the commoncharacteristic with an email message already stored in the second queue.13. The delivery server of claim 9 wherein the controller attempts tosend messages from each of the queues in the hierarchy at successivelydecreasing sending rates from the highest queue to the lowest queue. 14.The delivery server of claim 9 wherein the controller discards messagesfrom the lowest queue in the hierarchy after being stored therein forthe storage interval thereof.
 15. An electronic mail (email)communications method comprising: hosting a plurality of email messageboxes on at least one destination server; generating email messages eachassociated with a respective message box; storing the email messagesgenerated by the communications devices in a first queue; attempting tosend the stored email messages to the at least one destination server ata first sending attempt rate; moving email messages stored in the firstqueue to a second queue based upon receipt of a delivery failuremessage; attempting to send email messages stored in the second queue tothe at least one destination server at a second sending attempt ratethat is less than the first sending attempt rate; the second queue beingone of a plurality of queues arranged in a hierarchy, each queue in theplurality of queues having a storage interval that successivelyincreases from a highest queue to a lowest queue, the storage intervalbeing independent of the first and second sending attempt rates; movingemail messages from a higher queue to a next lower queue after beingstored in the higher queue for a duration of its storage interval; andmoving email messages having a common characteristic with a successfullydelivered email message to the first queue.
 16. The method of claim 15wherein the delivery failure message is based upon a failure to deliveran email message to a respective message box; and wherein the commoncharacteristic comprises a common message box.
 17. The method of claim15 wherein the at least one destination server comprises a plurality ofdestination servers; wherein the delivery failure message is based upona failure to deliver email messages to the destination servers; andwherein the common characteristic comprises having respective messageboxes hosted by a common destination server.
 18. The method of claim 15wherein storing further comprises storing directly in the second queueemail messages generated by the communications devices sharing thecommon characteristic with an email message already stored in the secondqueue.
 19. The method of claim 15 wherein attempting to send messagesstored in the second queue comprises attempting to send messages fromeach of the queues in the hierarchy at successively decreasing sendingrates from the highest queue to the lowest queue.
 20. The method ofclaim 15 further comprising discarding messages from the lowest queue inthe hierarchy after being stored therein for the storage intervalthereof.
 21. A non-transitory computer-readable medium havingcomputer-executable instructions for performing steps comprising:storing email messages generated by a plurality of communicationsdevices in a first queue, each email message having a respective messagebox associated therewith from among a plurality of message boxes hostedby at least one destination server; attempting to send the stored emailmessages to the at least one destination server at a first sendingattempt rate; moving email messages stored in the first queue to asecond queue based upon receipt of a delivery failure message;attempting to send email messages stored in the second queue to the atleast one destination server at a second sending attempt rate that isless than the first sending attempt rate; the second queue being one ofa plurality of queues arranged in a hierarchy, each queue in theplurality of queues having a storage interval that successivelyincreases from a highest queue to a lowest queue, the storage intervalbeing independent of the first and second sending attempt rates; movingemail messages from a higher queue to a next lower queue after beingstored in the higher queue for a duration of its storage interval; andmoving email messages having a common characteristic with a successfullydelivered email message to the first queue.
 22. The non-transitorycomputer-readable medium of claim 21 wherein the delivery failuremessage is based upon a failure to deliver an email message to arespective message box; and wherein the common characteristic comprisesa common message box.
 23. The non-transitory computer-readable medium ofclaim 21 wherein the at least one destination server comprises aplurality of destination servers; wherein the delivery failure messageis based upon a failure to deliver email messages to the destinationservers; and wherein the common characteristic comprises havingrespective message boxes hosted by a common destination server.
 24. Thenon-transitory computer-readable medium of claim 21 wherein storingfurther comprises storing directly in the second queue email messagesgenerated by the communications devices sharing the commoncharacteristic with an email message already stored in the second queue.25. The non-transitory computer-readable medium of claim 21 whereinattempting to send messages stored in the second queue comprisesattempting to send messages from each of the queues in the hierarchy atsuccessively decreasing sending attempt rates from the highest queue tothe lowest queue.
 26. The non-transitory computer-readable medium ofclaim 21 further comprising computer-executable instructions forperforming a step of discarding messages from the lowest queue in thehierarchy after being stored therein for the storage interval thereof.